1. Field of the Invention
The present invention relates to a method and apparatus for estimating the generated energy of a solar cell and, more particularly, to a method and apparatus for estimating the generated energy of an amorphous silicon solar cell.
2. Description of Related Art
A solar cell is affected by a change in weather such as solar radiation or temperature because of its nature and therefore generates power unstably. For this reason, in order to estimate generated energy depending on the set state of a solar cell, a simulation is conducted using solar radiation or temperature data. Especially in recent years, solar cells for general housing purposes have become popular. To design the necessary number of solar cell modules, generated energy at the site where the solar cells are set must be estimated.
A method of estimating generated energy is disclosed in "Guidebook for Design of Photovoltaic Power Generation System" (The OHM-Sha, Ltd.).
The basic formula for simulation of generated energy of a photovoltaic power generation system is obtained in the following way. Using a mean tilt solar radiation I.sub.s1, an overall correction coefficient K and rated power R1 of a solar cell are input. The generated energy of the photovoltaic power generation system is calculated using equation (1): EQU P2=I.sub.s1.multidot.K.multidot.R1 (1)
The overall correction coefficient K is rewritten using a temperature correction coefficient D1', a dust reduction correction coefficient D2, a power transmission loss correction coefficient D3, and an inverter correction coefficient D4: EQU K=D1'.multidot.D2.multidot.D3.multidot.D4 (2)
The temperature correction coefficient D1' in equation (2) is given by: EQU D1'=1+.alpha..sub.pmax (Tcm-Ts) (3)
where
.alpha..sub.pmax : -0.0037 (single-crystal solar cell) PA1 Tcm: mean monthly ambient temperature A1+15.degree. C. PA1 Ts: solar cell temperature under standard conditions=25.degree. C.
-0.0044 (polycrystalline solar cell) PA2 -0.0020 (amorphous silicon solar cell)
The power transmission loss correction coefficient D3 is given by equation (4) using an array unbalance loss E1, a wiring loss E2, and a diode loss E3: EQU D3=1-(E1+E2+E3) (4)
Assume that the above basic formula for calculating each monthly generated energy of the photovoltaic power generation system is used to estimate generated energy in a given area. As the mean monthly ambient temperature in that area increases, the temperature correction coefficient D1' is corrected in the negative direction. For this reason, when an amorphous silicon solar cell is used, the generated energy is estimated to be smaller than the actually generated energy. Conversely, the temperature correction coefficient D1' is corrected in the positive direction as the mean ambient temperature decreases. For this reason, when an amorphous silicon solar cell is used, the generated energy is estimated to be larger than the actual power. In the present invention, "amorphous silicon" includes "micro-crystallized silicon".
FIG. 2 is a graph showing generated energy estimated by the above method (solid line 11) and actually generated energy (broken line 12) for each month. A solid line 13 indicates the mean monthly ambient temperature. In a season when the mean monthly ambient temperature is particularly high or low, the difference between the estimated generated energy and the actually generated energy becomes large.
As is known, the amorphous silicon solar cell exhibits an optical degradation phenomenon due to its nature: the initial performance immediately degrades after the manufacture due to long-time outdoor exposure and finally stabilizes. The optical degradation in performance is a reversible phenomenon so annealing by heat allows recovery of the initial performance. This is called annealing recovery.
The solar cell generates power outdoors under a solar ray. The solar cell absorbs not only the solar ray necessary for power generation but also light components which do not contribute to power generation. The light energy which does not contribute to power generation is converted into heat and increases the temperature of the solar cell module. Actually, the temperature of the solar cell module during power generation is 20.degree. C. to 30.degree. C. in winter when the mean monthly ambient temperature is 2.degree. C. to 3.degree. C., and sometimes exceeds 60.degree. C. in summer when the mean monthly ambient temperature is 25.degree. C. to 26.degree. C. For this reason, as the ambient temperature or module temperature increases, the amorphous silicon solar cell recovers its performance by the above-described annealing recovery and generates more power. That is, the nature is reverse to that of a crystalline solar cell.
When the above-described basic formula for generated energy is used to estimate generated energy of a photovoltaic power generation system using an amorphous silicon solar cell, the temperature correction coefficient D1' is corrected in the negative direction as the mean monthly ambient temperature increases, and the generated energy is estimated to be small. As the mean monthly ambient temperature decreases, the temperature correction coefficient D1' is corrected in the positive direction, and the generated energy is estimated to be large. This increases the difference between the estimated generated energy and the actually generated energy for each month.